1. Field of the Invention
This invention relates to the field of MOS electrically programmable read-only storage devices which are capable of being erased by exposure to radiation.
2. Prior Art
Metal-oxide-semiconductors (MOS) electrically programmable read-only memories (EPROMs) have been utilized for the storage of information in the prior art. A cell for such memories is described in U.S. Pat. No. 3,996,657. These memories are erased by exposure to ultraviolet radiation. Memory arrays which employ such cells are disclosed in U.S. Pat. Nos. 3,728,695 and 3,744,036.
Redundant rows and columns have been utilized in the prior art for replacing defective rows and columns in read-only memories (ROMs) and random-access memories (RAMs). Fusible silicon links are used to enable these redundant elements. (A process for making a PROM employing fusible silicon links is described in U.S. Pat. No. 3,792,319.) U.S. Pat. No. 4,250,570 describes a memory employing redundant elements. Another redundancy system for memories is shown in Memory Redundancy Apparatus for Single Chip Memories, Ser. No. 320,600 filed Nov. 21, 1981 and assigned to the assignee of the present invention.
It is an object of this invention to provide a shield for integrated circuits with areas sensitive to radiation, some of which areas must be protected from radiation.
It is another object of this invention to provide an improved redundant memory circuit by shielding selected EPROM cells from radiation.